Pipe lay system with tension compensator

ABSTRACT

A pipe lay vessel with a reel for carrying a length of pipe and first drive part for controlling rotation of the reel. A substantially vertical guide structure includes at its upper end a deflection member for deflecting the pipe from a first trajectory extending between the reel and the guide structure to a second, substantially vertical trajectory. A clamping member carries the vertical pipe section and has second drive for feeding the pipe along the vertical trajectory. A movable tension compensator ( 7 ) is provided at the first pipe trajectory for lengthening or shortening the first pipe trajectory to maintain a substantially constant tension of the pipe along the first trajectory during unwinding of the reel. By displacing the tension compensator, the first pipe trajectory can be lengthened or shortened to compensate for variations in speed due to non-uniform winding of the pipe and varying diameter of the spooled pipe and to compensate for sagging of the pipe due to its own weight and length variations upon unspooling in view of occuring plasticity and partial straightening of the pipe.

[0001] The invention relates to a pipe lay vessel with a reel forcarrying a length of pipe and first drive means for controlling rotationof said reel, a substantially vertical guide structure comprising at itsupper end a deflection member for deflecting the pipe from a firsttrajectory extending between the reel and the guide structure to asecond substantially vertical trajectory, and a clamping member forcarrying the vertical pipe section having second drive means for feedingthe pipe along the vertical trajectory.

[0002] Such a vessel is known from WO-96/35902 in which a pipe isspooled from a storage reel to a bending shoe mounted on a derrickstructure that is situated above a moon pool. The vertical pipe sectionis passed via a 3-point straightening assembly into the water. The knownJ-lay vessel is particularly suitable for pipe laying in deep water. Itis known to suspend the vertical pipe section from a clamping device ortensioner, which can be driven or braked to feed the vertical pipesection towards the seabed. The reel supplying the pipe is also drivenor braked in a controlled manner.

[0003] Variations in pipe speed along the first trajectory between thereel and the deflection member will occur because the pipe coming fromthe reel will have a varying speed, even at constant reel rotation sincethe pipe is not always evenly spooled on the reel and the diameter ofthe unspooled pipe on the reel varies during the unspooling process.Also during start up and stopping and during emergency crash stopsituations, speed variations of the pipe will occur along the firsttrajectory and along the vertical trajectory where the pipe is fedthrough the tensioners. Speed variations may also be caused byplasticity of the pipe and partial straightening upon unspooling.Because of the varying tension along the first trajectory between thereel and the deflection member, sagging of the pipe or overload ofstructural elements that guide the pipe may occur.

[0004] It is therefore an object of the present invention to provide apipe lay system with which the tension of the pipe along the firsttrajectory can be maintained substantially constant. It is a furtherobject of the present invention to provide a system in which differencesin speed of the pipe section coming from the reel and the pipe sectionfed along the vertical trajectory can be compensated.

[0005] It is again a further object of the present invention to providea pipe lay system having a relatively simple and reliable compensatorsystem. Again, it is a further object of the present invention toprovide a pipe lay system in which a pipe straightener is provided whichis not affected by the tension compensation system and to provide a pipestraightener which can accommodate pipes of different diameter.

[0006] Thereto the pipe lay system of the present invention ischaracterised in that that a movable tension compensator is provided atthe first pipe trajectory for lengthening or shortening the first pipetrajectory to maintain a substantially constant tension of the pipealong the first trajectory during unwinding of the reel

[0007] By displacing the tension compensator, the first pipe trajectorycan be lengthened or shortened to compensate for variations in speed dueto non uniform winding of the pipe and varying diameter of the spooledpipe and to compensate for sagging of the pipe due to its own weight andlength variations upon unspooling in view of occurring plasticity andpartial straightening of the pipe. By the movable tension compensator,the tension in the pipe along the first trajectory (the back tension)can be maintained substantially constant.

[0008] In one embodiment of the pipe lay system according to the presentinvention, a detector is placed near the tension compensator formeasuring its position relative to an equilibrium position. A controlunit receives position signals from the detector and supplies a controlsignal to the drive means of the reel and/or to drive means of theclamping member for changing the pipe laying speed such that the tensioncompensator is at least substantially returned to its equilibriumposition. When the tension in the pipe section between the reel and thedeflection member becomes too large, the tension compensator will movesuch that the first trajectory is shortened. The control unit may eitherincrease the unwind speed of the reel, such that the tension compensatoris moved back to its equilibrium position or may lower the speed atwhich the pipe is fed along the vertical section. Similarly, whensagging of the pipe occurs, the tension compensator is moved such thatthe length of the first trajectory is increased. Decreasing theunspooling speed of the reel or increasing the pipe speed along thevertical section will result in the tension compensator returning to itsequilibrium position.

[0009] Detecting the position of the tension compensator may be carriedout optically or by encoders measuring a hinge angle of the compensator.The latter can occur when the tension compensator comprises a curved armextending transversely to the vertical guide structure and beinghingingly attached in a hinge point, remote from the guide structure.The end of the curved arm opposite the hinge point may be attached to afluid cylinder, which is extended or compressed when the tensioncompensator moves away from its equilibrium position. In this case,measuring the pressure in said cylinder or measuring its stroke willprovide a signal indicative of the deflection of the tension compensatorfrom its equilibrium position.

[0010] In one embodiment according to the present invention, a pipestraightener element is provided on the hinging arm of the tensioncompensator, the pipe straightener element having a curved surface and aradius of curvature substantially originating in the hinge point. Inthis way the point of departure of the vertical pipe section from theupper pipe straightener element to the lower two pipe straightenerelements can remain constant such that the pipe straightening moments inthe pipe straightener remain unaffected by movement of the tensioncompensator.

[0011] Analysis of the shape of the pipe before entering the pipestraightener has taught the applicant that the curvature of the pipe inthe region of the straightener varies for relatively large diameterpipes (diameters of about 40 cm or more) whereas Correlatively smalldiameter pipes (diameters of 10 cm or less) the curvature upon entryinto the straightener is more or less constant. Especially for largerdiameter pipes it is advantageous that at least one of the pipestraightener elements comprises a closed track around two rollers, atleast one adjustable roller being placed between the two rollers foraltering the width of the track. By controlling the track width of thepipe straightener, for instance by hydraulic cylinders or by screwspindles, a proper straightening for each pipe diameter can be obtainedin combination with the movable tension compensator according to thepresent invention.

[0012] An embodiment of the pipe lay system according to the presentinvention will be explained in detail with reference to theaccompanying, non-limiting drawings. In the drawings:

[0013]FIG. 1 shows a perspective view of a pipe lay vessel and J-laytower according to the present invention,

[0014]FIG. 2 shows a schematic sideview of the pipe storage reel, theJ-lay tower and the tension compensator of the present invention,

[0015]FIG. 3 shows the tension compensator on an enlarged scale, and

[0016]FIGS. 4a-4 c show the tension compensator and pipe straightenerfor the equilibrium position, upon-overtensioning of the pipe and uponslackening of the pipe, respectively.

[0017]FIG. 1 shows a pipe lay vessel 1 having a reel 2 and a verticalJ-lay tower 3. This vessel is especially suitable for pipe laying indeep water such as up to 2500 metres. On the reel 2, which may have adiameter of 30 metres, a steel pipe, which has been welded on shore isspooled. The pipe may have a length of 50 km and may weigh 1.500 tons.Instead of a hard pipe, a flexible pipe may also be spooled on reel 2.The pipe is fed along a first trajectory 4 to the J-lay tower 3 andpasses from thereon vertically downward to the seabed along a secondtrajectory 5. The pipe passes over a deflection member 6 which accordingto the present invention comprises a tension compensator having a curvedarm or pipe aligner 7 which is with a first end part 8 hinginglyattached to a horizontal support arm 9 of a frame 10. The second endpart 11 of the arm 7 can move relative to the J-lay tower 3 around thehinge point at the first end part 8.

[0018] The reel 2 is driven by a schematically indicated drive means 12.It is, however also possible to unwind the reel 2 by the weight of thepipe length depending from the J-lay tower and to control rotation ofthe reel by engaging a brake with a braking surface of the reel 2. Inthe J-lay tower 3, the vertical pipe length is carried by tensioners 13,which clampingly engage the pipe along the vertical trajectory 5.

[0019] According to the present invention, a detector 14 is providednear the pipe aligner 7 of the tension compensator to measure theposition of curved pipe aligner 7. On the basis of this position adetection signal is generated and passed to a control unit 15 whichcontrols the drive motor 12 (or the brakes) of the reel 2 and/or thetensioners 13. The detector 14 may comprise an optical detector or anencoder measuring the rotation of the pipe aligner 7 around first endpart 8. In a preferred embodiment, the detector 14 is provided by asensor connected to a cylinder that is attached to a second end part 11of the hinging arm 7, measuring the internal pressure or the stroke ofthe cylinder.

[0020]FIG. 2 schematically shows J-lay tower 3, the tensioner 13 and apipe straightener 16. The tensioner 13 comprises four clamping members17, 18, 19, 20, which in the configuration shown in FIG. 2, are closedand engage the vertical pipe section. Each clamping member 17-20comprises spaced-apart rollers 21, 22 around which a steel track isplaced in a closed loop. The track can be driven around the rollers 21,22 to feed the pipe along the vertical pipe trajectory 5. Oppositeclamping members 17, 19 and 18, 20 can be moved apart to disengage thepipe. Control unit 15 can control the speed of rotation of the beltaround the rollers 21, 22 of the clamping members 17-20 or can controlthe distance of opposite clamping members 17, 19 and 18, 20 from thepipe, such that the speed at which the pipe travels past the clampingmembers 17-20 can be adjusted.

[0021] The pipe straightener 16 comprises three pipe straighteningmembers 25, 26, 27. The lower pipe straightening members 26, 27 can bemoved perpendicularly towards or away from the vertical pipe section.The pipe straightening members 25, 26, 27 each comprise a steel trackwhich is placed in a closed loop configuration around two spaced-apartrollers, as can be seen more clearly in FIGS. 4a-4 c.

[0022]FIG. 3 shows the frame 10 carrying the curved arm of the pipealigner 7. The first end part 8 of a transverse arm 32 of the pipealigner 7 is hingingly attached to the horizontal support arm 9 of theframe 10 in hinge point 30. The second end part 11 of the transverse arm32 is attached to a hydraulic cylinder 31 which has its upper endattached to the J-lay tower 3. Depending on the weight (diameter) of thepipe on the reel 2, the pressure of the hydraulic cylinder is set toobtain a predetermined back tension in the pipe along trajectory 4. Whenthe tension in the first pipe section increases, the cylinder 31 will beextended and the curved arm of the pipe aligner 7 will hinge downwardlyaround hinge point 30. When the tension in the pipe decreases, thecylinder 31 will lift the second end part 11 of the curved arm of thepipe aligner 7. The pressure in the cylinder 31 or the position of thepiston rod can form an input signal for the control unit 15, thecylinder functioning as the detector 14, shown in FIG. 2. Although theinvention is described in FIG. 3 using a hydraulic cylinder 31, otherlifting devices, such as a counter weight or an electric drive motor maybe used.

[0023]FIG. 4a shows the tension compensator of the deflection member 6according to the present invention in its equilibrium position. Thefirst pipe straightening member 25 is attached to second end part 11 ofthe transverse arm 32 of the deflection member 6. The first pipestraightening member 25 comprises two spaced-apart rollers 33, 34 and asteel track 35 placed in a closed loop around the rollers 33, 34.Central rollers 36 define a curved surface 37 having a radius ofcurvature located at the hinge point 30. Thereby, upon hinging of curvedarm of the pipe aligner 7 around the hinge point 30, such as shown inFIGS. 4b and 4 c for a downward and upward direction respectively, thecurved surface 37 moves along the vertical pipe trajectory 5 while thepoint of departure 38 of the pipe from the curved surface 37 remains ata constant position with respect to pipe straightening members 26, 27.Thereby, the moments exerted by the pipe straightening members 25, 26,27 remain constant and the pipe straightening operation is independentof the position of tension compensator.

[0024] In the preferred embodiment, the middle pipe straightening member26 comprises spaced-apart rollers 40, 41 and a closed steel track 42.The central rollers 43 are each provided with adjustment members 44 suchas hydraulic adjustment members or screw spindles, to move the centralrollers 43 inwardly or outwardly with respect to the rollers 40, 41. Inthis way, the pipe straightening member 26 can be adjusted toaccommodate pipes with different diameters. Thereby, especially largediameter pipes having a diameter of for instance 40 cm can bestraightened properly by overbending to cause plastic deformation inorder to ensure proper pipe straightening.

1. Pipe lay vessel (1) with a reel (2) for carrying a length of pipe andfirst drive means (12) for controlling rotation of said reel (2), and asubstantially vertical guide structure (3) comprising at its upper end adeflection member (6) for deflecting the pipe from a first trajectory(4) extending between the reel (2) and the guide structure (3) to asecond, substantially vertical trajectory (5), a clamping member (13)for carrying a vertical pipe section having second drive means (17, 18,19, 20) for feeding the pipe along the vertical trajectory (5),characterised in that a movable tension compensator (7) is provided atthe first pipe trajectory (4) for lengthening or shortening the firstpipe trajectory (4) to maintain a substantially constant tension of thepipe along the first trajectory during unwinding of the reel (2). 2.Pipe lay vessel (1) according to claim 1, characterised in that itcomprises a detector (14, 31) for measuring the position of the tensioncompensator (7) relative to an equilibrium position, a control unit (15)for receiving a position signal from the detector (14, 31) and forsupplying a control signal to the drive means (12) of the reel (2)and/or to the second drive means (17, 18, 19, 20) for changing the pipelaying speed such that the tension compensator (7) is at leastsubstantially returned to its equilibrium position.
 3. Pipe lay vessel(1) according to claim 1 or 2, characterised in that the tensioncompensator (7) comprises a curved arm extending transversely to thevertical guide structure (3) and being hingingly attached to said guidestructure in a hinge point (30).
 4. Pipe lay vessel (1) according toclaim 3, characterised in that the hinge point (30) is located on an endpart (8) of the curved arm remote from the guide structure (3), whichend part (8) is connected to a support arm (9) extending transversely tothe guide structure (3), the end of the curved arm near the guidestructure (3) being connected to said guide structure (3)via a liftingdevice (31).
 5. Pipe lay vessel (1) according to claim 3 or 4,characterised in that a pipe straightener element (25) is provided onthe curved arm (7) near the vertical guide structure (3), the pipestraightener element (25) having a curved surface (37) with a radius ofcurvature substantially originating in the hingepoint (30).
 6. Pipe layvessel (1) according to any of the preceding claims, three pipestraightener elements (25, 26, 27) being provided along the verticalpipe trajectory (5), at least one pipe straightener element (26)comprising a closed track (42) around two rollers (40, 41), at least oneadjustable roller (43) being placed between the two rollers (40, 41) foraltering the width of the track.
 7. Pipe lay vessel (1) according toclaim 6, characterised in that the central pipe straightener element(26) comprises a track (42) which can be adjusted in width.